1. Field of the Invention
The present invention relates to a lens barrel having a plurality of advanceable and retreatable frame members for supporting photography lenses.
2. Description of the Related Art
In recent years, as zoom lenses have become increasingly longer in focal length, lens barrels have had to accommodate increasing amounts of advancing and retreating of lens groups. A zoom lens barrel proposed under such circumstances and disclosed in the Japanese Unexamined Patent Publication No. 2-89012 is adapted to drive first and second lens group supporting frames for larger amounts of advancing and retreating movements thereof by rotating a drive ring. Further, a first cam groove for moving the first lens group supporting frame back and forth and a second cam groove for moving a drive cam back and forth are provided in the drive ring. The second lens group supporting frame is driven through the drive cam to move back and forth.
However, in the zoom lens barrel disclosed in the Japanese Unexamined Patent Publication No. 2-89012, the first lens group supporting frame is directly driven by the drive ring to move back and forth, while the second lens group supporting frame is driven through the drive cam by the drive ring to move back and forth. Therefore, the advancing and retreating movements of the drive cam are about as much as those of movement of the first lens group supporting frame. Consequently, it is difficult to obtain large amounts of the lens groups. Further, this zoom lens barrel is not necessarily advantageous in the accuracy of the relative position between the first and second lens group supporting frames owing to the presence of the drive cam having a large lift amount.